Semiconductor devices such as Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are commonly used as power devices in applications, such as automotive electronics, power supplies, telecommunications, which applications require devices to operate at currents in the range of tenths up to hundreds of amperes (A).
Conventionally, by applying an appropriate voltage to the gate electrode of a MOSFET device, the device is turned on (i.e. in an on state) and a channel will be formed connecting the source and the drain regions allowing a current to flow. Once the MOSFET device is turned on, the relation between the current and the voltage is nearly linear which means that the device behaves like a resistance. The resistance is referred to as the on-state resistance Rdson.
Typically, MOSFET devices with low on-state resistance Rdson are preferred as they have higher current capability. It is well known that the on-state resistance Rdson may be decreased by increasing the packing density of a MOSFET device i.e. the number of base cells per cm2. For example, a hexagonal MOSFET (HEXFET) device comprises a plurality of cells, each cell having a hexagonal polysilicon gate and source and body regions forming vertices of the hexagonal polysilicon gate, and has a high packing density e.g. 105 hexagonal cells per cm2. Usually, the smaller the size of the cells, the higher is the packing density and thus, the smaller the on-state resistance. Therefore, many improvements to MOSFET devices are aimed at reducing the size of the cells.
When the device is turned off (i.e. in an off state), the voltage blocking capability is limited by the breakdown voltage. For high power applications, it is desirable to have a high breakdown voltage, for example, at least 200 volts.
European patent no. EP 1387408 discloses a Insulated Gate FET (IGFET) device in which the doping concentration in the epitaxial layer 11 between the source 5 and drain 3 regions does not vary substantially. The result is a ‘T’ shaped current flow 7 as shown in FIG. 1 between the source and drain regions during the on state of the IGFET device. The current path 7 extends generally parallel to the surface 9 of the epitaxial layer 11 from the source region 5 in the channel 13 and only extends vertically to the drain region 3 from the central part of the channel 13. The width of the current path is therefore restricted by the width of both the channel 13 and the drift region (which is the vertical part of the conductive path through the epitaxial layer 11) which limits the Rdson of the device. If the doping concentration of the epitaxial layer 11 is increased, the Rdson of the device will be increased but the breakdown voltage of the device will be decreased.
In order to increase the voltage capability of a MOSFET device, it is known to form a lightly doped drift region in the epitaxial layer between the drain region and the channel of a MOSFET device. The lightly doped drift region lowers the maximum electric field that develops around the PN junction formed between the body region 15 and the epitaxial layer when the device is in an off state and thus, ensures a higher breakdown voltage. However, reducing the doping in the drift region between the source and drain, increases the on-state resistance of the device.
There are other examples where techniques are employed to increase the breakdown voltage of a MOSFET device but which result in the increase in the on-state resistance Rdson. Thus, there is a trade-off between reducing Rdson and having a high enough break down voltage BVdss.
A need exists for improving the blocking voltage capability of a MOSFET device by increasing the device's breakdown voltage which does not compromise the on-state resistance Rdson.
U.S. Pat. No. 6,747,312 discloses a vertical MOSFET device in which an additional N-type region is formed between the source regions so as to increase the doping concentration in the channel. In addition, additional p-type regions are formed below the body regions of the MOSFET device. The additional p-type regions compensate for the increased doping of the additional N-type region so as to limit the impact on the breakdown voltage. However, further improvement in the trade off between Rdson and the breakdown voltage is desired. The additional regions of this MOSFET device are formed through thermal oxide layers and dedicated mask openings which significantly increases the cost of manufacturing the device.
There is therefore a need for an improved semiconductor device.